People have sought facile and reliable means to join flat flexible materials together to form such items as clothing, enclosures, bags, tents, balloons and the like. Early civilizations used crude forms of needles and threads to join materials together. More recently, sewing machines have provided a fast and reliable means to join two or more materials together. Sewing machines are known to provide a secure attachment between materials that is very strong and pleasing to the eyes. With the advent of adhesives, flat flexible materials can now be glued together to achieve a secure bond. One advantage that adhesives have over sewing is that a gas-tight and water-proof seal may be achieved. This type of seal is especially useful in the storage of food and in the manufacture of water-proof clothing. Unfortunately, adhesive type seals will degrade over a period of time and may break due to fluctuations in temperature and physical stress at the seal. Normal sewing techniques may provide a stronger connection between two materials; however the needle will leave pin holes in the material thus preventing the possibility of a gas tight seal.
With the introduction of thermoplastic and polymeric materials, attempts have been made to fuse two or more of these type materials together by applying pressure to the areas desired to be joined, and then applying heat to those pressure points until the materials melt together, creating a melt bond. Melt bonds provide a good seal that is gas-tight and water proof and has a reasonable amount of strength associated therewith. However, unless precise controls are maintained with respect to the fusing process variables, a poor seam may result. For example, if uneven pressure is applied to the materials to be fused, a weak or intermittent seal will be created, thereby affecting the quality of the seal.
Prior art has attempted to control the many variables involved in seam fusing thermoplastic materials together. In particular, U.S. Pat. No. 4,555,293 by French teaches the use of equipment that requires selecting the proper dimensions for the heating elements to be placed in a specific type die, the die being mateable with a matching recess in an opposite die, wherein the dimensions of the heating elements, recess and mating dies must be correlated to the thickness and compression characteristics of the thermoplastic sheet material to be bonded. Although useful, the French patent is limited in several respects.
In particular, the French patent only discloses the ability to thermobond two similar thermoplastic materials together. Additionally, the French apparatus is limited in only bonding thermoplastic materials, whereas the present invention is able to bond dissimilar polymeric materials together.
Those skilled in the art will appreciate that a thermoplastic material is limited to those materials having the property of softening when heated and becoming rigid again when cooled, without undergoing any appreciable chemical change to the material itself. In contrast, a polymeric material, of which thermoplastic material is a subset, is classified as a natural or synthetic chemical compound or mixture of compounds formed by a chemical reaction in which two or more small molecules that contain repeating structural units of the original molecules and that have the same percentage composition as the small molecules are lined together to form a stable material. Some polymeric materials, such as rubber, will undergo appreciable chemical change if heated past their melting points and then cooled. A polymeric material can either be woven, like a cloth, or non-woven, like a film.
A further disadvantage of current thermoplastic bonding machines is that inconsistent seam strength seals are generated. In other words, current seam fusing processes employ imprecise control systems which provide packages with inconsistent seam strength. For example, when opening a bag of potato chips at a top seam, the seam usually pulls open along its length and does not tear down the side. However, inconsistent seam strength may force the consumer to pull harder on the top seam and inadvertently tear open the side of the package, thereby spilling the contents. This problem is averted by more precisely controlling the seam strength.
An additional drawback of current thermoplastic bonding machines is that the heating element used to seam the food container is heated at a constant temperature throughout the packaging process. As such, this heat migrates to the contents of the package and creates undesired food deposits along the length of the area to be seamed. As such, leaks within the package are more likely to occur, thereby spoiling the contents of the package more quickly.
It will also be appreciated that current packaging materials require polymeric materials with multiple dissimilar layers of polymeric material to achieve the various properties of a good package. For example, one layer of material is typically used to create an air tight seal, while other layers are employed to provide structural strength and integrity to the package. Unfortunately, these multiple layer materials are expensive and, if improperly sealed, do not perform to the required expectations.
Accordingly, there is a need in the art for an apparatus and method for electronically seam fusing multiple layers of dissimilar polymeric materials to form a molecular bond that is stronger than the individual materials. Furthermore, there is a need in the art for an apparatus and method for electronically seam fusing multiple layers of dissimilar polymeric materials that provide a consistent seam strength and that can reduce the expense of using multiple polymeric layered materials.